Vfd control for multiple circuit refrigeration system

ABSTRACT

A refrigeration system includes a first refrigerant circuit having a first compressor, and a second refrigerant circuit having a second compressor, the second refrigerant circuit separate from the first refrigeration circuit. A variable frequency drive (VFD) is in signal communication with the first compressor and the second compressor and is configured to enable a variable speed operation of the first and second compressors. A controller is in signal communication with the VFD. The controller is programmed to selectively switch the first and second compressors between a variable speed operation using the VFD and a fixed speed operation such that in a first mode the first compressor is operated in the fixed speed operation and the second compressor is operated in the variable speed operation, and in a second mode the second compressor is operated in the fixed speed operation and the first compressor is operated in the variable speed operation.

FIELD OF THE INVENTION

The subject matter disclosed herein relates to refrigerant systems and, more specifically, to control of compressors for refrigerant systems associated with transport containers having multiple compartments.

BACKGROUND

Variable speed control of a compressor in a refrigeration or air conditioning application is often accomplished using a variable speed drive. This allows the removal of all unloading hardware from the compressor system. In typical applications involving more than one compressor, such as multiple circuit chillers, multiplexed compressor chillers, refrigeration, compressor racks, a variable speed control could be used with each compressor to selectively unload compressors as necessary based on system demand. Variable speed drives are expensive and therefore, multiple compressor systems requiring multiple variable speed drives also become extensively more expensive. In addition, the use of multiple speed drives adds to the complexity and space requirements of the air conditioning or refrigeration system.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, a refrigeration system is provided. The refrigeration system includes a first refrigerant circuit having a first compressor, and a second refrigerant circuit having a second compressor, the second refrigerant circuit separate from the first refrigeration circuit. A variable frequency drive (VFD) is in signal communication with the first compressor and the second compressor and is configured to enable a variable speed operation of the first and second compressors. A controller is in signal communication with the VFD. The controller is programmed to selectively switch the first and second compressors between a variable speed operation using the VFD and a fixed speed operation such that in a first mode the first compressor is operated in the fixed speed operation and the second compressor is operated in the variable speed operation, and in a second mode the second compressor is operated in the fixed speed operation and the first compressor is operated in the variable speed operation.

In addition to one or more of the features described above, or as an alternative, further embodiments may include: wherein the refrigeration system only comprises a single VFD; wherein the first compressor and the second compressor are the only compressors in signal communication with the single VFD; a transport container comprising a first cargo compartment and a second cargo compartment, wherein the first refrigerant circuit is thermally coupled to the first cargo compartment and the second refrigerant circuit is thermally coupled to the second cargo compartment; wherein the first cargo compartment is a frozen cargo compartment and the second cargo compartment is a perishable cargo compartment; wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation when a predetermined condition occurs; wherein the predetermined condition is when a temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature; wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature; and/or a first sensor disposed in the first cargo compartment and a second sensor disposed in the second cargo compartment, the first and second sensors configured to send respective signals to the controller indicative of at least one condition in the respective first and second cargo compartments, wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation based on the signals.

In another aspect, a method is provided of controlling a temperature in a first compartment and a second compartment with a refrigeration system comprising a first refrigerant circuit having a first compressor, a second refrigerant circuit having a second compressor, a variable frequency drive (VFD) in signal communication with the first compressor and the second compressor, and a controller in signal communication with the VFD. The method includes operating, with the controller and the VFD, the refrigeration system in a first mode such that the first compressor is operated at a variable speed and the second compressor is operated at a fixed speed, monitoring the first and second compartments for the occurrence of a predetermined condition, and switching, with the controller and the VFD, upon the occurrence of the predetermined condition, the refrigeration system to operate in a second mode such that the second compressor is operated at a variable speed and the first compressor is operated at a fixed speed.

In addition to one or more of the features described above, or as an alternative, further embodiments may include: operating only one of the first and second compressors at the variable speed during each of the first and second modes; switching between the first and second modes with only a single VFD; wherein the step of monitoring the first and second compartments comprises monitoring a first sensor and a second sensor respectively disposed in the first compartment and the second compartment for the occurrence of a predetermined condition; wherein the predetermined condition is when a monitored temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature; and/or wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic illustration of an exemplary refrigeration system; and

FIG. 2 is a schematic illustration of an exemplary variable frequency drive that may be used with the refrigeration system shown in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a simplified schematic diagram of an exemplary container refrigeration system 10 operatively associated with a transport container 12 having multiple temperature controlled compartments or zones defined therein. For example, as illustrated, transport container 12 includes a first compartment 14 and a second compartment 16. However, transport container 12 may have more than two compartments.

Refrigeration system 10 is configured to provide a cooling capacity to temperature controlled compartments 14, 16, which may store perishable commodities and/or frozen or chilled products. Transport container 12 may include, for example, truck trailers for road transport, railroad cars, and shipping container bodies for land and sea service. Refrigeration system 10 may be secured to a portion of transport container 12 such as a wall thereof.

Refrigeration system 10 generally includes a first refrigerant circuit 20, a second refrigerant circuit 30, a variable frequency drive (VFD) 40, and a controller 50. First refrigerant circuit 20 generally includes a compressor 22, a condenser 24, an expansion device 26, and an evaporator 28. Second refrigerant circuit 30 is fluidly separate from circuit 20 and generally includes a compressor 32, a condenser 34, an expansion device 36, and an evaporator 38. First refrigerant circuit 20 is operably associated with first compartment 14 and is configured to provide cooling demand thereto. Similarly, second refrigerant circuit 30 is operably associated with second compartment 16 and is configured to provide cooling demand thereto.

Operation of first and second refrigerant circuits 20, 30 is similar. For example, operation of circuit 20 may begin at compressor 22 where suction gas (refrigerant) is compressed to a higher temperature and pressure. The refrigerant gas is supplied to condenser 24 where it is cooled and condensed by a heat transfer fluid such as air flowing across condenser 24. By removing latent heat, the gas condenses to a high pressure, high temperature liquid and flows to expansion device 26 (e.g., an expansion valve).

The liquid refrigerant passing through expansion device 26 is expanded or reduced in pressure and may be flashed or vaporized into a gas. The expanded refrigerant is subsequently supplied to evaporator 28 where it is further vaporized against a heat transfer fluid such as return air. The transport container air to be cooled is passed across tubes/fins of evaporator 28, and heat is removed from the air as it is absorbed by the refrigerant in evaporator 28. This causes the temperature of the low pressure refrigerant vapor to increase before it is returned to compressor 22 to complete the refrigeration cycle.

Refrigeration system 10 may be charged with a relatively low critical temperature refrigerant such as carbon dioxide (R744). Because carbon dioxide has a low critical temperature, most refrigeration systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime. In systems operating in a transcritical cycle, condensers 24, 34 function as gas coolers rather than as condensers and operate at a refrigerant temperature and pressure in excess of the refrigerant's critical point, while evaporators 28, 38 operate at a refrigerant temperature and pressure in the subcritical range.

VFD 40 selectively controls the capacity of compressors 22, 32 by varying the speed at which the compression devices operate by varying the frequency of AC electric current supplied to a drive motor of the compressor (not shown). VFD 40 receives electric current at line frequency, selectively alters that frequency, and outputs the DC current to the drive motor at the selected frequency. In the transport refrigeration application described herein, the power source comprises an electric current generator 42 driven by an internal combustion engine 44, such as the engine of a truck or trailer tractor associated with transport container 12 or a separate internal combustion engine dedicated to driving electric current generator 42.

Controller 50 is in signal communication with VFD 40, one or more sensors 52 operably associated with first compartment 14, and one or more sensors 54 operably associated with second compartment 16. As used herein, the term controller refers to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.

In some embodiments, sensors 52, 54 are temperature sensors configured to respectively sense an internal ambient temperature of cargo compartments 14, 16. As such, controller 50 may monitor the temperatures within compartments 14, 16 and adjust operation of refrigeration system 10 to meet a desired cooling demand within compartments 14, 16, as described herein in more detail. Alternatively sensors 52, 54 may be refrigerant temperature sensors thermally coupled to, for example, evaporator 38.

Controller 50 is programmed to control VFD 40 to selectively switch compressors 22, 32 between a variable speed operation and a fixed speed operation such that only one compressor 22, 32 is operated at a variable speed while the other compressor 22, 32 is operated at a fixed speed. Controller 50 determines which compressor requires the most cooling capacity at any given time (e.g., based on signals from sensors 52, 54), and subsequently enables VFD 40 to switch variable speed operation between compressors 22, 32. Accordingly, controller 50 is programmed to selectively switch refrigeration system 10 between a first mode where compressor 22 is operated at variable speed and compressor 32 and any remaining compressors (not shown) are operated at fixed speed, and a second mode where compressor 32 is operated at variable speed and compressor 22 and any remaining compressors (not shown) are operated at fixed speed.

Controller 50 may be programmed to switch the operation of compressors 22, 32 between the fixed and variable speed operations when various predetermined conditions occur. For example, first compartment 14 may be a deep frozen compartment and second compartment 16 may be a perishable products compartment that has a higher setpoint temperature than frozen compartment 14. Controller 50 may be programmed to operate in the first mode with compressor 22 in a variable speed operation and compressor 32 in a fixed speed operation. Controller 50 may subsequently automatically switch the compressor operation into the second mode when sensor 52 and/or 54 senses a predetermined condition (e.g., deep frozen first compartment 14 achieves a temperature set point). In some embodiments, refrigeration system 10 may only include two compressors 22, 32 and a single VFD 40.

Controller 50 may be programmed to switch the operation of compressors 22, 32 between the fixed and variable speed operations when other predetermined conditions occur. For example, predetermined conditions may include a predetermined system pressure, a predetermined system temperature, a predetermined temperature in each of compartments 14, 16, a predetermined ambient temperature, a predetermined user setpoint (time, temperature, and/or pressure), a box temperature differential (e.g., difference between the compartment temperature and the ambient, or difference between supply air and return air), based on refrigeration system properties (e.g., temperature, pressure), when within a predetermined percentage of a user setpoint, when one compartment has a greater pull-down than another, compressor protection (low mass flow, high suction temperature, suction liquid slugging), and/or noise reduction.

FIG. 2 illustrates one embodiment of VFD 40. In the exemplary embodiment, VFD 40 generally includes a housing 60, a VFD control board 62, a first relay 64, a second relay 66, a third relay 68, a fourth relay 70, a first set of output terminals 72, and a second set of output terminals 74. Relays 64, 66, 68, and 70 are selectively activated by controller 50 to selectively operate compressor 22, 32 in one of the variable speed operation and the fixed speed operation. As illustrated, compressor 22 is operated in the variable speed operation and compressor 32 is operated in the fixed speed operation. In this configuration, relays 64, 66 are not energized, and VFD control board 62 provides a current to output terminals 72 via relays 64, 66. VFD control board 62 determines compressor speed based on controller demand.

A compressor contactor 76 is utilized to aid a start/stop control mode in order to shut down compressor 22 when the box setpoint has been met. Start/stop mode is a selectable configuration that will shut down compressor 22 when the box setpoint has been met, facilitating saving on load draw from the generator 42 and fuel savings from the engine 44. Compressor contactor 76 may be external or internal to VFD housing 60, or could be controlled via VFD control board 62.

Relays 68, 70 are energized and a power source 78 provides power to compressor 32 via fourth relay 70 and output terminals 74. Power source 78 may be an AC current power electrical source such as generator 42, or an outside power source such as house power or electrical source during ferry transport. Compressor terminals 4-6 of compressor 32 are shorted at third relay 68, which causes compressor 32 to run at the fixed speed. A compressor contactor 80 is utilized to aid a start/stop control mode in order to shut down compressor 32 when the box set point has been met.

Described herein are systems and methods for controlling the cooling capacity of multiple compartments of a multi-temperature transport container. The systems include a single VFD selectively switching two or more compressors between a variable speed operation and a fixed speed operation. The compressor associated with a compartment requiring the most cooling capacity is operated in the variable speed operation while the remaining compressor(s) are operated at the fixed speed operation. Accordingly, only a single VFD is used rather than using a dedicated VFD for each compressor in the system. As such, the systems described herein reduce total product cost, weight, and fuel consumption while requiring less space and providing capacity requirements needed for each compartment's temperature control.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A refrigeration system comprising: a first refrigerant circuit having a first compressor; a second refrigerant circuit having a second compressor, the second refrigerant circuit separate from the first refrigeration circuit; a variable frequency drive (VFD) in signal communication with the first compressor and the second compressor, the VFD configured to enable a variable speed operation of the first and second compressors; and a controller in signal communication with the VFD, the controller programmed to selectively switch the first and second compressors between a variable speed operation using the VFD and a fixed speed operation such that in a first mode the first compressor is operated in the fixed speed operation and the second compressor is operated in the variable speed operation, and in a second mode the second compressor is operated in the fixed speed operation and the first compressor is operated in the variable speed operation.
 2. The refrigeration system of claim 1, wherein the refrigeration system only comprises a single VFD.
 3. The refrigeration system of claim 2, wherein the first compressor and the second compressor are the only compressors in signal communication with the single VFD.
 4. The refrigeration system of claim 1, further comprising a transport container comprising a first cargo compartment and a second cargo compartment, wherein the first refrigerant circuit is thermally coupled to the first cargo compartment and the second refrigerant circuit is thermally coupled to the second cargo compartment.
 5. The refrigeration system of claim 4, wherein the first cargo compartment is a frozen cargo compartment and the second cargo compartment is a perishable cargo compartment.
 6. The refrigeration system of claim 4, wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation when a predetermined condition occurs.
 7. The refrigeration system of claim 6, wherein the predetermined condition is when a temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature.
 8. The refrigeration system of claim 6, wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature.
 9. The refrigeration system of claim 4, further comprising a first sensor disposed in the first cargo compartment and a second sensor disposed in the second cargo compartment, the first and second sensors configured to send respective signals to the controller indicative of at least one condition in the respective first and second cargo compartments, wherein the controller is programmed to switch the first and second compressors between the variable speed operation and the fixed speed operation based on the signals.
 10. A method of controlling a temperature in a first compartment and a second compartment with a refrigeration system comprising a first refrigerant circuit having a first compressor, a second refrigerant circuit having a second compressor, a variable frequency drive (VFD) in signal communication with the first compressor and the second compressor, and a controller in signal communication with the VFD, the method comprising: operating, with the controller and the VFD, the refrigeration system in a first mode such that the first compressor is operated at a variable speed and the second compressor is operated at a fixed speed; monitoring the first and second compartments for the occurrence of a predetermined condition; and switching, with the controller and the VFD, upon the occurrence of the predetermined condition, the refrigeration system to operate in a second mode such that the second compressor is operated at a variable speed and the first compressor is operated at a fixed speed.
 11. The method of claim 10, further comprising operating only one of the first and second compressors at the variable speed during each of the first and second modes.
 12. The method of claim 10, further comprising switching between the first and second modes with only a single VFD.
 13. The method of claim 10, wherein the step of monitoring the first and second compartments comprises monitoring a first sensor and a second sensor respectively disposed in the first compartment and the second compartment for the occurrence of a predetermined condition.
 14. The method of claim 13, wherein the predetermined condition is when a monitored temperature in at least one of the first and second cargo compartments reaches a predetermined setpoint temperature.
 15. The method of claim 13, wherein the predetermined condition is when an ambient temperature reaches a predetermined temperature. 